Melamine is well-known as a cross-linking agent for polymers. Melamine, which is crystalline in its pure form, is typically reacted with formaldehyde and an alcohol to form an alkoxylated melamine resin. This resin can then be used as a cross-linking agent for a variety of polymers, such as hydroxyl group-containing polymers (e.g., hydroxyl acrylics, polyesters, hydroxyl urethanes). In this type of cross-linking reaction, the carbon atom that is alpha to the amino nitrogen on the melamine reacts with the hydroxyl group on the polymer in an acid-catalyzed reaction with the melamine's alkoxy substituent leaving as an alcohol.
Melamine resins have been used successfully as cross-linking agents for a variety of polymer systems, including hydroxyl acrylic polymer systems used in the automotive coatings area, and particularly for automotive clearcoats. They offer the advantage of being used in the so-called "one-pack" systems, where the cross-linkable coating composition can be coated as one formulation. However, when melamine cross-linked hydroxyl acrylic polymers are used in automotive clearcoat coatings, the resulting finish suffers from a phenomenon known as environmental etch. Environmental etch appears as milky or cloudy marks on clearcoat finishes that have been exposed to the elements.
There is thus a need in the art for a weather-resistant one-pack cross-linkable coating composition that uses a cross-linking agent other than the standard melamine aldehyde resin.